Generally, in a high input and high luminance discharge lamp, and the like, an emitter is added to a cathode to facilitate the electron emission. Thorium oxide is conventionally used for the emitter. However, thorium is a radioactive substance and has various restrictions in handling. Thus, use of rare earth elements and compounds thereof as an alternative substance of thorium has been proposed. The rare earth element is a substance that has a low work function (“work function” generally means energy required when an electron is released outward from the interior of the substance) and that excels in electron emission. The rare earth element is expected to serve as an alternative substance of thorium.
Japanese Patent Application Laid-Open Publication No. 2005-519435 (Patent Literature Document 1) discloses a discharge lamp in which lanthanum oxide (La2O3), hafnium oxide (HfO2), zirconium oxide (ZrO2), or the like is additionally contained, as an emitter, in tungsten, which is the material of the cathode.
The rare earth oxide such as lanthanum oxide (La2O3), however, has a higher vapor pressure than thorium oxide (ThO2), and thus vaporizes relatively easily. Therefore, if the rare earth oxide is used, in place of the thorium oxide, for the emitter to be contained in the cathode, the rare earth oxide may be excessively vaporized by the lighting lamp, and a situation of early depletion may arise. If the emitter is depleted, the electron emission function in the cathode is lost, thus causing flickers and shortening the life of the lamp.
The emitter contained in the cathode is less likely to be rapidly transported from the rear portion toward the front end of the cathode, and hence the fact that the emitter actually contributing to the electron emission only exists at the front end of the cathode may be one reason for the emitter depletion.
Currently, therefore, the discharge lamp that uses the emitter substance other than thorium still suffers from problems such as the lighting becoming unstable at an early stage. Particularly, in the high input discharge lamp of greater than or equal to 1 kW, it is significant that the early vaporization of rare earth elements and barium-based substances results in unstable lighting of the discharge lamp.
Japanese Patent Application Laid-Open Publication No. 2002-141018 (Patent Literature Document 2) discloses a cathode structure in which an alkali earth metal (oxide) is used for an emitter substance. FIG. 15 of the accompanying drawings shows such cathode structure. Specifically, an easy electron emission part 81 added with alkali earth metal oxide, as an emitter, is embedded in a cathode 80, and exposed at a front end of the cathode.
The alkali earth metal oxide, which is the emitter, is exposed to an arc in such structure as well, and the vaporization of the emitter is facilitated. This is similar to the cathode described in Patent Literature Document 1. As a result, similar problems arise, particularly at the front end of the cathode, in that the emitter is depleted at an early stage and the electron emission function in the cathode is lost. This causes flickers and shortens the longevity of the lamp.